Add test cases for bugs 1869746 and 1873518

Tests 1 and 2 (negative):
- Try to create an incremental backup from a backup
  previously created by another tenant (admin/user).

Test 3 (positive):
- Create backups as different tenants and confirm their
  incremental backups follow the respective parents.

Change-Id: I1d7896d9d7e20f7e6ac816af379f59cc86a3daa6
Depends-on: Icb621ff5966133f59d9d43ca2dd9f8e1919b1149
2 files changed